Legion
If you think you have one rat, you have 20. If you think you have 20 you have a thousand. etc
I lived in a house last year with many, many rats. We rather hopefully referred to this massive infestation as The Rat, or Mr Rat. Sightings were frequent and always shocking.
I posoined, I'm afraid. Tried trapping but with no success and setting the trap was terrifying.
The poison worked, but you have to keep at it for a while, refilling the (pet-proof) tray a few times.
I don't know your rat welfare policy, so sorry if this offends.